OROGRAPHIC FACTORS 



269 



Flow of soil, or solifluction, occurs mostly upon impermeable or 

 frozen ground and is favored by repeated freezing and thawing of the 

 superficial water-soaked "sUde layer" (Gleitschicht) (Frodin, 1917; 

 Nordhagen, 1928, p. 536). 



The rubble flow in the limestone ranges of the Alps (Braun- 

 Blanquet, 1926, p. 229) is also due to the steepness of the slopes but is 

 not to be confused with soHfluction. The more easily movable earth 

 and rock particles are carried by rain and snow water along definite 

 paths determined by the structure of the soil, forming tonguelike 

 ridges of rubble as much as a meter high and slowly moving forward. 

 They often considerably affect the minor topographic features. Tufts 



Fig. 137. — Manner of distribution of the various epiphyte forms according to 

 exposure near the base (within 1 m.) of 100 poplars at Zurzach on the Rhine; showing 

 the pattern for foliose lichens (1), crustose lichens (2), Radula form (3), Orthotrichum 

 form (4), and for Leucodon form (5). {After Ochsner.) 



of Dryas and Carex firma take an outstanding part in the vegetation of 

 these formations. 



In the subtropics, even in the southwest Alps and the Pyrenees 

 and the southern Rocky Mountains, the vegetation on the steeper 

 slopes is exposed to washouts and floods due to violent rainfall (see 

 p. 115). 



In the high mountains of central and southern Europe the steepness 

 of the slopes is synonymous with a brief duration of the snow cover. 



The vegetation on the slope awakens much earlier than that of the 

 level places, not only because of the earlier melting of the snow but 

 also because it often slides down the valley in avalanches. Avalanches 

 also keep extended areas on steep slopes free from forest growth and 



